I heard the version with Lord Nelson.
Sailing off the coast of Spain in the early 1800's, the sailor up in the crows nest calls down "French frigate off the port bow". Lord Nelson asks his aide to bring him his scarlet shirt. "Why, m'Lord" the confused aise asks.
"That way if I am wounded, the blood won't show, and bring comfort to the enemy"
Just then the chap in the crows nest calls down "Entire French fleet off the starboard bow"
"And also bring me my brown trousers"
Cheers,
Mike